Use the other eye when the tracker loses one, and add a headset fit check

SteamVR's combined gaze keeps going on one eye, but it holds the lost
eye's yaw, so the gaze moves half as far sideways as the eyes do.
ft-gaze now reads each eye's tracking uncertainty and raw measurement
from eye-server.mmap. When the tracker loses an eye, ft-gazed takes the
gaze from the other one, plus the offset that eye usually shows
against both, learned while both are seen. On a recording, one eye
alone came out a median 0.8 degrees from both eyes' gaze.

Glances down at the keyboard, past every screen, aren't sent. The
pointer stays put, and eyes lost there don't count as lost.

The gaze probe gets a Headset fit mode. It shows per-eye tracking,
openness and confidence, maps where each eye gets lost, gives hints,
and has a guided check. The settings app opens it from the Gaze page
and shows how often each eye is lost. The probe can also test each eye
alone, and its side panel now collapses to a title bar so the dot
isn't hidden behind it.

Snapping to UI elements is deferred; the mouse drag is the correction.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/design.md

@@ -83,7 +83,7 @@ A few overlays need special handling:
 
 Head follow is experimental and off by default. It works, but it's only lightly tested, and the feel is mostly a matter of its settings; polishing it is left open. With it on (`POINTER_FOLLOW=1`, or a mouse button mapped to Head follow on/off), the cursor rides on a reference direction, where you were facing when your head last settled, and keeps its offset from it. The mouse can put the cursor anywhere up to `POINTER_FOLLOW_REACH` (70 degrees) from the reference, a corner of your view included. While your head stays within `POINTER_LEASH_DEG` of the reference, nothing moves on its own. Once your head has been past the leash for `POINTER_LEASH_DELAY` (0.2 s, so a glance out and back doesn't count), the reference eases to where you're facing (time constant `POINTER_LEASH_RETURN`, 0.2 s), never falling further behind than the leash, and the cursor ends up back where it was in your view. Then it waits for the leash again. Two earlier versions didn't work out. Moving the reference only while your head pulled at the end of the leash left it up to the leash off after you turned back, and getting it centred again meant overshooting with your head. Easing it toward your facing all the time moved the cursor on every small head movement. A leash of 0 makes the reference your facing direction, so the cursor is locked to your view, and mouse movement shifts it within the view. Head roll is ignored, so tilting your head doesn't swing the cursor around. While the left button is held the cursor stays put in the room, so your head can't nudge a click or a drag. When you let go, it carries on from where it is instead of jumping.
 
-Gaze mode is experimental and off by default (`POINTER_GAZE=1`, the Gaze page of Frametop Input Settings, `gaze/ft-gazectl on`, or a mouse or controller button mapped to Gaze pointer on/off). It's MAGIC pointing (Zhai, Morimoto and Ihde, 1999): the pointer goes where you look, and the mouse does the last bit. The gaze service (`gaze/ft-gazed`) sends the helper the corrected gaze at 90 Hz, and while the gaze has the pointer, the cursor ray is that gaze from the eye. The pointer is aimed at the gaze each frame, not steered toward it, so nothing can pile up. An earlier try in the gaze probe steered the pointer with relative moves, and lost it when the pointer went idle or a controller had the laser. Moving the mouse takes the pointer from the gaze. A left press while the gaze has the pointer isn't sent at once: the pointer stops where the gaze put it, you drag it onto what you meant with the button still down (panels only see it hover), and the release clicks there. Clicking at once clicked wherever the gaze was, often the wrong thing, before you could correct it. A press held still for `POINTER_GAZE_HOLD` (0.5 s) becomes a real press, so drags still work: hold, then move. Outside games the pointer then stays: the mouse going idle doesn't release it. A moving controller still releases it, as without gaze; the mouse is gaze mode's only pointer device for now. The dot shows only while the mouse moves it (`POINTER_GAZE_SHOW`), while a press is held, and as a pulse for each click; otherwise it's transparent, so the laser still lands on it. The gaze moving the pointer doesn't show it: you know where you're looking. Looking more than `POINTER_GAZE_RETAKE` (5 degrees) away from it, with the mouse still, gives it back, so small eye movements around the pointer don't pull it off what you're doing. A mouse nudge of up to `POINTER_GAZE_NUDGE_MAX` (8 degrees) before a click is sent to the gaze service as a lesson: you were looking at where you clicked when the mouse took over, so the nudge is the eye tracker's error there. Using it is what calibrates it. See `gaze/README.md` for the service, the calibration, and what was measured.
+Gaze mode is experimental and off by default (`POINTER_GAZE=1`, the Gaze page of Frametop Input Settings, `gaze/ft-gazectl on`, or a mouse or controller button mapped to Gaze pointer on/off). It's MAGIC pointing (Zhai, Morimoto and Ihde, 1999): the pointer goes where you look, and the mouse does the last bit. The gaze service (`gaze/ft-gazed`) sends the helper the corrected gaze at 90 Hz (from one eye while the tracker has lost the other), and while the gaze has the pointer, the cursor ray is that gaze from the eye. The pointer is aimed at the gaze each frame, not steered toward it, so nothing can pile up. An earlier try in the gaze probe steered the pointer with relative moves, and lost it when the pointer went idle or a controller had the laser. Moving the mouse takes the pointer from the gaze. A left press while the gaze has the pointer isn't sent at once: the pointer stops where the gaze put it, you drag it onto what you meant with the button still down (panels only see it hover), and the release clicks there. Clicking at once clicked wherever the gaze was, often the wrong thing, before you could correct it. The drag is the correction. Snapping the pointer onto buttons and links is deferred: it needs accessibility (AT-SPI) on in the Frametop session, where it's off (no registry runs), plus app restarts, and it makes Chromium and Electron apps use more CPU. A press held still for `POINTER_GAZE_HOLD` (0.5 s) becomes a real press, so drags still work: hold, then move. Outside games the pointer then stays: the mouse going idle doesn't release it. A moving controller still releases it, as without gaze; the mouse is gaze mode's only pointer device for now. The dot shows only while the mouse moves it (`POINTER_GAZE_SHOW`), while a press is held, and as a pulse for each click; otherwise it's transparent, so the laser still lands on it. The gaze moving the pointer doesn't show it: you know where you're looking. Looking more than `POINTER_GAZE_RETAKE` (5 degrees) away from it, with the mouse still, gives it back, so small eye movements around the pointer don't pull it off what you're doing. A mouse nudge of up to `POINTER_GAZE_NUDGE_MAX` (8 degrees) before a click is sent to the gaze service as a lesson: you were looking at where you clicked when the mouse took over, so the nudge is the eye tracker's error there. Using it is what calibrates it. See `gaze/README.md` for the service, the calibration, and what was measured.
 
 Replacing a loaded driver's files, as re-running the installer used to do, leaves SteamVR honoring the virtual controller's hand role but not its laser claim: the dashboard pointer stays unassigned until SteamVR restarts. The driver installer now leaves an unchanged driver in place.
 
@@ -121,6 +121,8 @@ Steam, not systemd, suspends the Frame: after `system_idle_suspend_ac_sec` (an h
 
 Flatpak apps need `XDG_DATA_DIRS` to include Flatpak's exports, or Plasma opens Discover instead of launching them, so the session sources `/etc/profile.d/flatpak.sh`.
 
+The private runtime directory also moves the session's document portal to `$XDG_RUNTIME_DIR/frametop/doc`, and that broke saving and uploading in Flatpak apps. The file picker (xdg-desktop-portal 1.18.4 on SteamOS) gives a sandboxed app the host path of the file it picked, `/run/user/1000/frametop/doc/ID/NAME`. Inside the sandbox the portal is at `/run/flatpak/doc`, and `/run/user/1000` is a private per-app folder (`.flatpak/APP/xdg-run` in the runtime directory). So Brave created the missing folder there, "finished" the download into it, and the file vanished when the session cleaned up. The session script now links that path to `/run/flatpak/doc` in each installed app's folder before Plasma starts. Upstream xdg-desktop-portal fixed this after 1.22.1 (commit `69ba5e1`) by handing Flatpak apps `/run/flatpak/doc` paths, after which the links go unused.
+
 A podman container's monitor process (conmon) stays in the cgroup of whatever started the container, and `distrobox enter` starts it on demand. When a Frametop service happened to start the `dev` container, stopping that service stopped the container and everything in it, including the desktop's compositor. `scripts/container-up.sh` starts the container in a systemd scope of its own before anything enters it.
 
 Program names stay within 15 characters, because Linux truncates process names there and the scripts find programs with `pgrep -x` and `pkill -x`. That's why the prefix is `ft-`.